The invention relates generally to wireless communications systems and, more particularly, to control efficient indication of a suitable location to perform a handoff from a first antenna to a second antenna.
Wireless communications systems provide communication services using geographically dispersed Base Station Transceiver Systems (BTSs) comprising at least one antenna, but typically two, three, or more. Each antenna is capable of transmitting and receiving within a limited geographical service area and may be configured such that the service area is directional or generally concentric. The service area for a BTS is commonly referred to as a cell or a sector. Moreover, a plurality of BTSs are generally coupled to a Base Station Controller (BSC), and, likewise, a plurality of BSCs are generally coupled to a Mobile Switching Center (MSC). The MSC provides interfaces to the Public Switched Telephone Network (PSTN) and the Packet Data Switched Network (PDSN), and, together with the BTSs and BSCs, form a wireless network that provides wireless communications services within a service area.
Wireless communication systems may utilize any communication protocol, one of which is the Code Division Multiple Access (CDMA) protocol as defined by the TIA/EIA-95-B and the CDMA 2000 standards. CDMA is a technology employing low-powered radio transmission that utilizes a broad spectrum broadcast to ensure quality of the transmission.
The capacity of a CDMA system is dependent upon, among other things, the number of distinct mobile stations (MSs) that may be communicating with a BTS over a single carrier frequency. In an attempt to increase capacity, CDMA systems are typically deployed with multiple carrier frequencies overlaying a single service area. With overlaying frequency coverage, some MSs are serviced on one of the carrier frequencies while other MSs are serviced on other carrier frequencies. Moreover, any particular BTS or antenna typically provides communication services on a subset of the available carrier frequencies, resulting in a situation in which adjacent service areas may utilize different carrier frequencies.
As a result of the limited geographic coverage of an antenna and the differing carrier frequencies between service areas, the MS is frequently commanded to handoff from a first antenna to a second antenna when moving from the service area of the first antenna to the service area of the second antenna. If the MS communicates user data, such as voice transmissions, with the first and second antennae, both of which are controlled by a single BSC, on the same carrier frequency, the handoff is commonly referred to as a soft handoff. During soft handoffs, an MS generally communicates user data with the first antenna and the second antenna simultaneously to enhance the transmission before dropping the first antenna. The convention in this application is to refer to soft handoff as both, separately and in combination, the process and state of the second antenna being added to the mobile active set of the MS. The process of adding and removing an antenna from the mobile active set of the MS is well known to one of ordinary skill in the art and will not be discussed in further detail.
Furthermore, if the MS is: (1) required to switch to a different carrier frequency; (2) required to switch to an antenna or BTS controlled by a different BSC that is controlled by a same MSC (intra-MSC/inter-BSC); or (3) required to switch to an antenna or BTS controlled by a different BSC and a different MSC (inter-MSC/inter-BSC), the handoff is commonly referred to as a hard handoff. It is well known to one of ordinary skill in the art that during hard handoffs the MS generally only communicates user data with one antenna, i.e., the communication of user data generally switches instantaneously from the first antenna to the second antenna. The process of switching from one antenna to the second antenna is well known to one of ordinary skill in the art and will not be discussed in further detail.
When performing a hard handoff, the MS is typically commanded to handoff from a first antenna to a second antenna when the signal strength of the second antenna exceeds a predetermined threshold. One particular method of assisting the MS in a hard handoff from the first antenna to the second antenna is by the use of a pilot signal. Pilot signals are typically transmitted by either the BTS as part of a CDMA forward channel, which comprises a pilot channel, a synch channel, a paging channel, and a traffic channel, or by a pilot beacon unit, whose transmission generally comprises a pilot channel and a synch channel. The second BTS generally transmits the pilot signal if the first antenna and BTS utilize the same carrier frequency as the second antenna and BTS. A pilot beacon unit generally transmits the pilot signal if the first antenna and BTS utilize a different carrier frequency than the second antenna and BTS. The source of the pilot signal will be referred to as being transmitted by an antenna, regardless of whether the pilot signal originates from the BTS or pilot beacon unit. Furthermore, a pilot beacon unit, which is preferably co-located with the second antenna, is generally required if the carrier frequencies of adjacent antennae differ. The use of a pilot signal and a pilot beacon unit are well known to a person of ordinary skill in the art and, therefore, will not be discussed in further detail, except as required to describe the present invention.
More particularly, handoffs are generally performed when the pilot signal strength of the second antenna, measured as the ratio of energy per chip to the noise spectral density of the total received forward-link interference, commonly referred to as Ec/I0, is equal to or exceeds a predetermined system parameter, and may occur in either an idle-state or a traffic-state. Generally, the MS continually monitors the strength of the pilot signal from the second antenna, PS2(Ec/I0), and compares PS2(Ec/I0) to the value of two system parameters.
The CDMA standards typically provide two system parameters, T_ADD and T_COM, both of which are used to assist in hard and soft handoffs. The MS typically receives and stores T_ADD and T_COM from the BTS and/or BSC. The MS requests handoff from a first antenna to a second antenna when the strength of the pilot signal from the second antenna, PS2(Ec/I0), is greater than or equal to T_ADD. Furthermore, the MS also requests handoff from a first antenna to a second antenna when the difference between the strengths of the signals from the second and the first antenna, PS2(Ec/I0)xe2x88x92PS1(Ec/I0), is greater than or equal to T_COM*0.5 dB.
For instance, an MS currently communicating with a first antenna having a pilot signal strength of PS1(Ec/I0), monitors the signal strength of nearby antennae to determine when handoff is appropriate. A second antenna transmits to the MS a pilot signal with a strength of PS2(Ec/I0). The MS compares PS2(Ec/I0) with the system parameter T_ADD, and compares the difference of PS2(Ec/I0) and PS1(Ec/I0) with the system parameter T_COM*0.5 dB. If either PS2(Ec/I0) is greater than or equal to T_ADD, or PS2(Ec/I0)xe2x88x92PS1(Ec/I0) is greater than or equal to T_COM*0.5 dB, the MS requests a handoff from the first antenna to the second antenna.
Unfortunately, the system parameters T_ADD and T_COM assist both soft and hard handoffs. As a result, it is difficult for the service provider to adjust the T_ADD and T_COM values to obtain optimal performance for both hard and soft handoffs simultaneously. For instance, if the service provider optimizes T_ADD and T_COM for soft handoffs, hard handoff situations are generally adversely affected. Likewise, if the service provider optimizes T_ADD and T_COM for hard handoffs, soft handoff situations are generally adversely effected.
Therefore, there is a need, for one or more reasons discussed or nevertheless existing, for a method of optimizing when handoffs, soft and hard, occur.
The present invention provides a method through which wireless communications service providers adjust when hard and soft handoffs occur individually relative to the signal strength of one or more antennae. The method preferably comprises using hysterisis parameters added as an offset to the handoff system parameters in the case of hard handoffs and using the handoff system parameters in the case of soft handoffs, thereby providing a mechanism to adjust hard and soft handoffs individually. Alternatively, the method comprises using hysterisis parameters added as an offset to the handoff system parameters in the case of soft handoffs and using the handoff system parameters in the case of hard handoffs, or a combination thereof.